Polymer products are used as components of imaging and photosensitive systems and particularly in photoimaging systems such as those described in Introduction to Microlithography, Second Edition by L. F. Thompson, C. G. Willson, and M. J. Bowden, American Chemical Society, Washington, D.C., 1994. In such systems, ultraviolet (UV) light or other electromagnetic radiation impinges on a material containing a photoactive component to induce a physical or chemical change in that material. A useful or latent image is thereby produced which can be processed into a useful image for semiconductor device fabrication.
Although the polymer product itself may be photoactive, generally a photosensitive composition contains one or more photoactive components in addition to the polymer product. Upon exposure to electromagnetic radiation (e.g., UV light), the photoactive component acts to change the rheological state, solubility, surface characteristics, refractive index, color, electromagnetic characteristics or other such physical or chemical characteristics of the photosensitive composition as described in the Thompson et al. publication.
For imaging very fine features at the submicron level in semiconductor devices, electromagnetic radiation in the far or extreme ultraviolet (UV) is needed. Positive working resists generally are utilized for semiconductor manufacture. Lithography in the UV at 365 nm (I-line) using novolak polymers and diazonaphthoquinones as dissolution inhibitors is a currently established chip technology having a resolution limit of about 0.35–0.30 micron. Lithography in the far UV at 248 nm using p-hydroxystyrene polymers is known and has a resolution limit of 0.35–0.18 nm. There is strong impetus for future photolithography at even shorter wavelengths, due to a decreasing lower resolution limit with decreasing wavelength (i.e., a resolution limit of 0.18–0.12 micron for 193 nm imaging and a resolution limit of about 0.07 micron for 157 nm imaging). Photolithography using 193 nm exposure wavelength (obtained from an argon fluorine (ArF) excimer laser) is a leading candidate for future microelectronics fabrication using 0.18 and 0.13 μm design rules.
Photolithography using 157 nm exposure wavelength (currently obtained from a fluorine excimer laser) is a leading candidate for future microlithography (beyond 193 nm) provided suitable materials can be found having sufficient transparency and other required properties at this very short wavelength.
A dissolution inhibitor (DI) may be utilized in a photoresist composition. Typically, a dissolution inhibitor is included in a photoresist composition to assist in the development process. A good dissolution inhibitor will inhibit the unexposed areas of the photoresist layer from dissolving during the development step in a positive working system. A useful dissolution inhibitor may also function as a plasticizer which function provides a less brittle photoresist layer that will resist cracking. These features are intended to improve contrast, plasma etch resistance, and adhesion behavior of photoresist compositions.
A variety of bile-salt esters (i.e., cholate esters) are known to be effective dissolution inhibitors for deep UV resists, beginning with work by Reichmanis et al. in 1983. (E. Reichmanis et al., “The Effect of Substituents on the Photosensitivity of 2-Nitrobenzyl Ester Deep UV Resists”, J. Electrochem. Soc. 1983,130,1433–1437.) Bile-salt esters are useful as DIs for several reasons, including their availability from natural sources, high alicyclic carbon content, and particularly their transparency in the deep and vacuum UV region, (which essentially is also the far and extreme UV region), of the electromagnetic spectrum (e.g., typically they are highly transparent at 193 nm). Furthermore, the bile-salt esters are also attractive DI choices since they may be designed to have widely ranging hydrophobic to hydrophilic compatibilities depending upon hydroxyl substitution and functionalization.
Some examples of bile-salt esters include cholic acid, deoxycholic acid, lithocholic acid, t-butyl deoxycholate, t-butyl lithocholate, and t-butyl-3-α-acetyl lithocholate. However, the ester groups that are present in these compounds tend to strongly absorb light at very short wavelengths which makes the bile-salt esters less desirable as dissolution inhibitors at wavelengths of less than 193 nm. Carefully controlling the quantity of the bile-salt ester and/or the photoresist layer thickness are possible approaches for overcoming the absorption characteristics of bile-salt esters when imaging at the shorter wavelengths. These approaches have disadvantages since a process parameter such as quantity control has associated costs and a thinner photoresist layer, as compared to a thicker layer, will often have a higher number of defects that arise during imaging.
A need still exists for dissolution inhibitors that are not only transparent at short wavelengths but compatible with the polymeric binders that are useful at the shorter wavelengths.
In the process of forming patterned microelectronic structures by means of lithography, it is common in the art to use at least one antireflective layer either beneath the photoresist layer, a BARC, or on top of the photoresist layer, a TARC, (or sometimes referred to simply as an ARC) or both. Antireflective coating layers have been shown to reduce the deleterious effects of film thickness variations, and the resulting standing waves caused by the interference of light reflecting from various interfaces within the photoresist structure, and the variations in the exposure dose in the photoresist layer due to loss of the reflected light. The use of an antireflective layer results in improved patterning and resolution characteristics of the photoresist materials because it suppresses reflection related effects.
A need also exist for antireflective coatings that have optical transparency at imaging wavelengths of 193 nm and or 157 nm.